Power supplies are required in most every electronic device used today. An example use of a power supply is in an Ethernet switch. Such power supplies often require multiple different power levels for various portions of the Ethernet switch. A given power level is sometimes referred to as a power rail. Such power supplies often receive one voltage level as an input, such as 12 volts and generate numerous different power levels through a plurality of power converters.
It is known that in such a power supply problems arise if the voltage levels generated by each of the power converters rise to their full level, or drop to zero upon termination, such that particular differential voltage levels between any two of the power rails reach undesired levels. Thus, techniques have been developed for both controlling the order at which power rails come up to power or terminate power, referred to as voltage sequencing, as well as controlling the relative voltage differential between any two power rails, referred to a voltage tracking. Various approaches include series element control, shunt element control, clamp diodes, enabling a lower voltage rail in response to a power good signal, and feedback based control. Each of these methods has disadvantages.
Series element control involves a voltage tracking controller, large series pass field effect transistors, and a set of multiple power supplies. The voltage tracking controller senses the output voltage and linearly controls the gate voltage to the series pass field effect transistors. This requires the series pass field effect transistors to be able to pass the entire load of the power rail with minimal voltage drop. This approach can be expensive, reduce reliability, generate heat, and consume board real estate.
Shunt element control involves monitoring a voltage delta between two rails and linearly controlling the gate from a field effect transistor shorting the two rails. The field effect transistor must be capable of handling large currents associated with shorting high current rails. Power is dissipated only during start up and shut down, unlike the series element method.
The use of clamp diodes involves simply attaching a Schottky diode between power rails. If the higher voltage rail falls below the lower rail, the load of the lower rail is increased to bring the rails down together. This approach has a problem of possible overloading of the lower voltage supply, causing undue stress. It also requires diodes large enough to clamp the rails together. The diodes are physically large and costly.
In the fourth approach, a power good signal may be used to enable a lower voltage rail. This solution is suitable for voltage sequencing, but not useful for voltage tracking. A large number of small logical elements is required.
In a feedback based control approach, a resistor divider is used off of a higher voltage rail to generate a reference for the lower voltage rail pulse width modulator. A lower voltage reference ramps up or down to follow the upper rail. This feature does not accommodate voltage margining.